Regular readers will know that I’ve been doing my best over the last several years to get through the temnospondyls of the world. Temnospondyli, for the one or two or you that don’t know, is an enormous and substantially diverse clade of anamniotes (‘amphibians’) that was an important and persistent presence between the Early Carboniferous [...]

Regular readers will know that I’ve been doing my best over the last several years to get through the temnospondyls of the world. Temnospondyli, for the one or two or you that don’t know, is an enormous and substantially diverse clade of anamniotes (‘amphibians’) that was an important and persistent presence between the Early Carboniferous and the Jurassic. Relict forms hung on into the Cretaceous, and many experts argue that modern amphibians – the lissamphibians – are the direct descendants of one specific temnospondyl lineage, the dissorophoids of the Carboniferous and Permian. Dissorophoids have been covered (briefly) on Tet Zoo before (see links below).

Among the several dissorophoid lineages are the dissorophids of North America, Russia and China, most often associated with Cacops from the Permian of Texas. Dissorophids and their close relatives the trematopids form the dissorophoid clade Olsoniformes (Anderson et al. 2008). While trematopids may have been mostly cryptic terrestrial animals, dissorophids were reasonably large predatory temnospondyls that must have been patrolling open environments alongside contemporaneous amniotes. Indeed, they were ‘amniote-mimicking temnospondyls’ in some respects. When illustrating them, remember that they didn’t have the moist, naked skin we so often see in life reconstructions: temnospondyls were scaly, their bellies, flanks, dorsal surfaces and limbs being covered variously in spindle-shaped, ovoid and sub-circular scales (Witzmann 2007). The ovoid scales were arranged in overlapping rows. [UPDATE: be sure to read the comments below. The scales may not have been all that visible in many of these animals, since they were embedded within, or overlain by, the epidermis.]

Notable dissorophid features include a row of armour plates arranged along the dorsal midline and an obvious otic notch, sometimes enclosed by bone on all sides and almost certainly housing a large tympanum (an ear drum). Robust and well ossified limbs, a lack of lateral line canals, a tall, box-like skull shape and other features indicate that these animals were strongly terrestrial. Their big tympana (and slender stapes) indicate that dissorophids were detecting airborne sounds before contemporaneous amniotes convergently evolved the same features (Reisz et al. 2009). Were they listening for approaching predators, or using vocalisations when sending signals, or both? On twitter (@TetZoo) I happened to mention the fact that I had an urge to draw some temnospondyls. This inspired noted anamniote worker Jason Pardo to say "then I probably ought not to suggest drawing a male Platyhystrix calling for females with a fully-inflated vocal sac", the idea behind his comment obviously being that these animals were indeed vocal and using their voices for sexual reasons. Hold that thought.

In Cacops and similar dissorophids, the median armour plates were fused to the tops of the neural spines. It’s often been suggested that these plates mostly functioned in defence, perhaps protecting these animals from the big predatory synapsids of the time, but suggestions that the plates served some sort of role in burrowing, that they reduced evaporative water loss, or that they reduced flexibility and stiffened the body have also been made (Dilkes & Brown 2007).

In some dissorophid taxa, long, laterally compressed neural spines evolved, their gnarly, rugose tips apparently representing the original armour scute. Hyper-elongate spines, presumed to have formed a sail-like structure superficially similar to the more familiar one on the Permian synapsid Dimetrodon, were present in Platyhystrix rugosus of the Lower Permian and Astreptorhachis ohioensis from the Upper Pennsylvanian. It’s been argued that these taxa should be separated from dissorophids and awarded their own ‘family’, the Platyhystricidae, but I don’t think this is useful given that there are intermediate forms with mid-length neural spines, such as some of the Aspidosaurus species. Anderson et al. (2008) found Dissorophidae to include distinct dissorophine and cacopine clades, with Ecolsonia cutlerensis being the sister-taxon to the clade formed by these two groups. Most recently, Schoch (2012) found Platyhystrix and Aspidosaurus to be outside the dissorophine + cacopine clade. This seemingly destroys (for now) the idea that Aspidosaurus is paraphyletic with respect to Platyhystrix: it might mean that tall neural species evolved more than once within dissorophids, or even - shock horror - that tall spines were primitive for the group, and lost in the ancestors of the dissorophine + cacopine clade.

Incidentally, I’m guilty here of perpetuating the idea that the spines of these animals were separate structures, webbed by skin. This is how everyone draws Platyhystrix (see the Bakker illustration below, and the Nobu Tamura one shown here), but it might be incorrect: there may have been skin webbing between some of the spines, but others were in such close contact that there couldn’t have been a ‘web’ of the sort we always see. Note that in the Astreptorhachis spines shown below, the spine apices are tightly pressed together, and this might have been true for at least some of the Platyhystrix spines as well – the fossils really aren’t complete enough to tell.

We don’t really know what these animals did with their long neural spines. Contrary to what it says in some popular books there’s no indication that the spines were associated with the high degree of vascularisation that you might expect for a thermoregulatory role, and Vaughn (1971) thought that they probably initially evolved to provide mechanical support during terrestrial locomotion (see also Dilkes & Brown 2007). I’m not a fan of the argument that sails of this sort are best explained as special thermoregulatory structures (even for animals like Dimetrodon, the "sail was for thermoregulation" idea is dodgy and far less convincing than widely thought): based on analogy with extant animals, I think a role in display was more likely the primary driver behind the evolution of these structures, though that doesn’t mean that a role in thermoregulation, self-defence or even camouflage is ruled out.

Anyway, the reason we’re here today is that I’ve been surprised by how few images there are online of Platyhystrix, one of the most bizarre and notable of all temnospondyls. Robert Bakker did one of the best reconstructions for his book The Dinosaur Heresies (Bakker 1986) and several post-1986 illustrations are clearly based on that one. I’ve mentioned the need for new Platyhystrix images on twitter and facebook, and in this article you can see the results. Thanks to Mike P. Taylor (of SV-POW) for the highly accurate and intricately detailed version he supplied, and thanks also to Tracy Ford, Ethan Kocak (of The Black Mudpuppy fame), Henrik Petersson and Maija Karala for the brilliant and innovative versions you can also see here. I did a version myself and colourised versions appear here courtesy of Tim Morris and Gareth Monger.

On the subject of the life appearance of Platyhystrix, it should be noted to begin with that Platyhystrix is not known from fantastic material: those distinctive neural spines have been found on a few occasions, but the rest of the skeleton remains poorly known. Skull material has been discovered on several occasions, with the most complete specimen being the crushed and fragmented one described by Berman et al. (1981). This specimen (AMNH 11545: shown below) suggests that the skull was about similar in form to that of better known dissorophids, but that the otic notch wasn’t enclosed by bone posteriorly.

Another peculiarity includes a so-called papillose or nodular dermal sculpturing: the surface of the skull is covered in low bumps and ridges that would presumably have given the live animal a knobbly skin texture. A similar texture is present on the neural spines and also on the ribs, suggesting that much of the animal’s dorsal surface was similarly knobbly. The presence of this nodular ornament isn’t unique to Platyhystrix, but it is unusually extensive in this taxon, being uniquely present across the cheek and orbital regions of the skull.

Platyhystrix also possesses a surprisingly high tooth count: it has as many as 65 simple, peg-like teeth on each side of the upper jaw. This isn’t unique within dissorophids (there are taxa with even higher numbers), but it certainly makes you wonder what, and how, these animals were grabbing and eating.

Note also that Platyhystrix is pretty big: the AMNH 11545 skull is over 19 cm long along the midline. This size is not exceptional for the species, with some fragments of skull coming from even larger animals (Berman et al. 1981). In Cacops and most other dissorophids the skull is usually about 12-14 cm long, so Platyhystrix might have been a large member of the group if it was proportioned like those other taxa (in Cacops, the skull is about 33% of total length, so a complete Platyhystrix might have been about 60 cm long). Fayella may have been Platyhystrix-like in size (Olson 1972).

So, with this impressive and wonderful assortment of Platyhystrix images now out there and online, let's hope that we see much more of this amazing temnospondyl in future. And much more on temnospondyls to come here... in time, of course. Oh, I nearly forgot the best image - the one provided by Mike Taylor. Here you go...

Witzmann, F. 2007. The evolution of the scalation pattern in temnospondyl amphibians. Zoological Journal of the Linnean Society 150, 815-834.

The views expressed are those of the author(s) and are not necessarily those of Scientific American.

ABOUT THE AUTHOR(S)

Darren Naish

Darren Naish is a science writer, technical editor and palaeozoologist (affiliated with the University of Southampton, UK). He mostly works on Cretaceous dinosaurs and pterosaurs but has an avid interest in all things tetrapod. His publications can be downloaded at darrennaish.wordpress.com. He has been blogging at Tetrapod Zoology since 2006. Check out the Tet Zoo podcast at tetzoo.com!

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